This file stores the hierarchy of values, much like a GConf database or the Windows Registry. In this file, element nodes correspond to keys (folders) and attributes to entries.

In future, there will be no guarantee that preferences are stored in an XML file.

Find a place for your value

When creating a new preference value, start by examining this file and finding a logical place for the new value. For example, if I want to add a value for some option related to the Select tool, I find the group element with id="tools" and inside I find <eventcontext id="select"/>. Now this element is empty, but I can store my value in a new attribute. If you have added a new object to the program, such as a new tool or dialog, add a new element in an appropriate top-level element. If you want to store something entirely new and not yet taken care of, you can even add a new top-level element.

This is because if you think about element nodes as folders and attributes as entries, you would get a path like "/options/nudgedistance/value", which is redundant. It's better to place your preference as an attribute of the options node. However, it would be ideal if you found a better place for your preference than the generic "options" hierarchy.

Some points of interest:

Element names are irrelevant. What matters for finding the value is the id attribute values. So you can use group elements or any others, whatever seems more logical.

Values are stored in attributes. No text within elements is allowed (except whitespace).

Add the value to the skeleton

When no preferences file exists, a new one is created based on src/preferences-skeleton.h. Moreover, if the preferences file lacks some values, the defaults are taken from the same file. So, I edit this file adding

Don't forget to escape quotes. The value given in this file is the default; if the user has changed it, his/her value in preferences.xml takes precedence over src/preferences-skeleton.h.

Now when you recompile, run, and exit Inkscape, the new elements with the default values will be added to your local preferences.xml (without affecting other values there).

Access the value in the program

Now for the interesting part. If you want to access your value in the program, start by adding

#include "preferences.h"

This header includes a singleton class that you can use to access the preferences. To get at the value of the preference, use its members methods. To receive an instance of this class, call the static function get(). Always name the pointer to this object "prefs", for consistency.

What this call does is it reads the value from the memory representation of preferences.xml. This representation was created on program start, may be used or modified by changing any values in it, and will be written back into preferences.xml when the program exits. Preferences are also saved in the crash handler, so changes to them should be preserved even if Inkscape crashes.

To pinpoint the value we need, you pass to this function a text string in a syntax that is vaguely reminiscent of XPath, except that it uses id attribute values instead of element names. That is, /tools/select/bar means, find top-level element with id="tools", within it, an element with id="select", and finally get the value from the bar attribute of the select node. The second argument is the default value which will be returned if no such value is stored in the preferences.

It is important that this prefs->getDouble call is done before each use of the value, and not just once upon program launch, because the preferences may be edited by the user via a dialog. If your preference is directly tied to some user interface element, it's best to use observers to update it (more on them later).

Similarly, to write a new value back into the preferences' memory representation, use

prefs->setDouble("/tools/select/bar", new_bar);

In addition to double values, you can also store and retrieve Glib::ustrings, ints, bools, CSS styles and a few more (check up-to-date Doxygen documentation for a list). Always choose the right type for your value.

Preference observers

If your preference is directly tied to something and you don't want to retrieve its value every time, you can use observers. You do this by deriving from the Inkscape::Preferences::Observer inner class. Here is an example on how to do it:

You can set observers on any point of the hierarchy - both keys and individual entries. This way, the notify method will be called every time your preference is updated. If you want to change the preference from its handler, be sure to guard against infinite recursion (or ideally don't do this, because it's not a good practice). Inkscape::Preferences::Entry has more public methods that essentially mirror those of the Preferences class, but don't take a path parameter. To get the preference's path, use getPath(). To get the preference's base name (i.e. the last element of its path), use getEntryName().

NOTE: In future, this mechanism may be obsoleted in favor of sigc++ signals.

Guard against screw-ups

You should not assume that the values in the prefs file were written there by your code - they may have been very well directly tweaked by the user. If your code will crash when an out-of-bounds value is returned, you should use the methods getIntLimited and getDoubleLimited. Do not use these to store boolean preferences as integers limited to 0 and 1 - use getBool and setBool for that.

Add UI

In ui/dialogs/inkscape-preferences.cpp is the code that attaches a GUI to any given preference. If your option doesn't already have a logical place to go, add it under the "Misc" section. For example, to add an integer-selecting spin-button (sb) you could add to inkscape-preferences.h the line

The next section of this tutorial will be devoted to how a preference value can be edited in an options dialog. See related page: PreferencesDialog.

Discuss below:

I also think we should consider having a global preferences file in /etc/inkscape/preferences.xml, and similarly with markers.svg, that are installed during program installation, and merged with or overwritten by the user's settings. This is standard UNIX config file strategy. - bryce

Njh wonders whether you would be better off using gconf?

Maybe, but I'm not in a position to decide. Right now I'm interested in how I can use the existing preferences system to store and retrieve a value. -- bb

Looking further, it appears that gconf is indeed a similar XML system, so at some point it should be easy enough to switch over. --njh

Mental and I were discussing GConf, and he brought up the issue of Win32 compatibility. It would be nice if we could abstract a preferences interface that would work with both the windows registry and GConf. I think they have similar interfaces - so it shouldn't be impossible --ted

Please do not use the Windows registry. The beauty of having the preferences file is that Inkscape does not need an install or admin rights, and does not litter the OS (at least, i think that is what the registry does). It is very easy for a user that has problems to delete his preferences.xml and download a new one; much easier than fiddling in the registry. - Johan

If this is going to be done, why not also look into implementing Apple's .plist support, it's a simple xml format defined: http://www.apple.com/DTDs/PropertyList-1.0.dtd
These files are stored in ~/Library/Preferences/tld.<company>.<product>.plist - tom

Well, if we're looking around at existing approaches, the discussion probably wouldn't be complete without taking a look at the new Java preferences API added in 1.4. They currently store to a dir/dir/xml tree on Linux, and to the registry on Windows, so they do that exact abstraction we're looking at. (funny thing is, for some apps, I wrote a layer to abstract that for running in Java VM's prior to 1.4 - double indirection). --jon

The prefs have been extensively refactored, so much of the above discussion is no longer relevant. Coding in plist support should be easier now if desired. GConf is also possible as a backend but it would be a pain for users that need to switch between KDE and Gnome, so on the mailing list we decided against it. On top of that, it seems like GConf will be obsoleted in favor of the (hopefully desktop-neutral) dconf in some near future. -- Krzysztof Kosiński (tweenk)